1. Field of the Invention
The present invention relates to turbochargers and more particularly to turbochargers for use on relatively small as well as large internal combustion engines.
2. Prior Art
Turbocharging is a means of greatly extending the power range and flexibility of internal combustion engines and has come to be an accepted practice, and in many cases a necessity, for heavy duty diesel engines of 200 or more horsepower. Turbocharging is also used to maintain power at increasing altitude, for instance in aircraft engines.
The effective application of a turbocharger to an internal combustion engine will normally increase power output 50-100 percent and reduce full load specific fuel consumption (sfc) by 50-10 percent. The reduced specific fuel consumption is attributed to two items. First, at a given speed the engine internal friction remains relatively constant even though the power output is increased considerably. This results in an effective improvement in mechanical efficiency. Second, if the efficiencies of the turbocharger components are high enough and the exhaust temperature of the engine on which the turbocharger is used is sufficiently high, there will result a positive pumping loop that adds to the net cycle output.
Turbocharging is normally thought of only as a means for increasing horsepower of decreasing full load specific fuel consumption. In the evolution of engines presently utilizing turbochargers, such as large heavy duty diesel engines, aircraft engines, racing engines and the like, this understanding has been adequate in that in the end application these engines are operated at or near full load for a large portion of the duty cycle. However, most applications do not require that the engine operate at or near full load for extended periods of time. In fact, in most applications, the engine generally is operated below 50% power and in many applications the engines operates well below 20% power during most of its operation. Examples of these applications are engines used in automobiles, light and medium trucks, generator sets, compressors, tractors, construction equipment and the like.
Engines operating at these low power settings are very inefficient. In a diesel engine, this inefficiency is a result of thermal efficiency decay as combustion temperature decreases and because the internal engine friction remains relatively constant regardless of load. In a gasoline engine, this inefficiency results from pumping loop loss increases with decreasing load and because the internal engine friction remains relatively constant regardless of load.
Thus, by utilizing a smaller "effective engine size," that is, an engine having a smaller displacement rate (the product of 1/2 displacement times engine speed for a four cycle engine) by either a reduction in displacement, a reduction in operating speed, or a combination of both, the part load fuel consumption may be improved. In gasoline engines, this improvement results from reduced engine friction and reduced pumping loop losses. In diesel engines, this improvement is a result of reduced engine friction and a higher thermal efficiency due to higher combustion temperatures.
In many applications, turbocharging may be used to permit the use of engines having smaller "effective engines size." By turbocharging, it is relatively easy to obtain twice the naturally aspirated power per cubic inch of displacement and in some cases three times the power. However, attempts to turbocharge smaller "effective engine sizes" have generally been unsuccessful. This failure can generally be attributed to the present design of turbochargers which are built around a journal bearing and a flat disc type thrust bearing. This type bearing system requires from one to three horsepower (depending upon the particular turbocharger and the speed required in the application) just to overcome friction. While this loss may be somewhat insignificant in applications where turbines are required to develop in excess of thirty horsepower for the compression process (typically engines of 200 or more horsepower), it becomes very significant when turbocharging engines having less than 100 horsepower. For example, where the turbocharger turbine power is 60-80 horsepower, a bearing friction loss of 2-3 horsepower is insignificant. However, in a smaller turbocharger where the turbine power is only 15 horsepower a bearing friction loss of 2-3 horsepower, or more likely 4-5 horsepower due to the higher rpm at which the smaller turbochargers are operated, represents almost a third of the total turbine horsepower produced and is completely unacceptable.
The presently used bearing systems also require considerable radial and axial clearances to provide for oil flow and rotor stability. These clearances are translated into a relatively large clearance over the blading of the compressor and turbine rotors thereby affecting the efficiency of both the compressor and turbine. For example, the journal and disc thrust bearings, commonly used in present day turbochargers, may require a clearance of 0.015 inches between the turbine and compressor blades and the surround structure. Where the blade height is 1 inch, the clearance to blade height ratio is only 11/2 percent. However, where a smaller turbocharger is desired, having blade heights of 0.2 inches, a clearance of 0.015 inches between the blades and surround structure amounts to 71/2 percent of the blade height. Therefore, where an 0.015 inch clearance is acceptable in larger turbocharger applications, it becomes completely unacceptable when smaller turbochargers are being designed. Therefore, in smaller turbochargers, this clearance becomes more and more critical to the overall performance of the turbocharger and ultimately to the performance of the engine.
The bearing system now being used in turbochargers are lubricated with engine oil, although all bearing failures are the result of contaminated engine oil or lack of engine oil pressure during starts. Where high speed journal bearings are used in a conventional turbocharger, continuous oil flow is inherently required to provide shaft stability as well as to carry away heat generated by viscous friction. Oil flow is also required to carry away the heat transferred into the bearing system from the adjacent turbine (which operates at temperatures as high as 1600 degrees F. ). Even if antifriction ball bearings were substituted for the journal bearings in conventional turbochangers, a continuous oil flow would be required to carry away heat transferred from the turbine. Thus, while lubrication is a necessity for the proper operation of present day turbochargers, lubrication also accounts for many of the failures. Further, continuous oil flow lubrication requires substantial plumbing and associated structure for providing the lubricant to the bearings.
Moreover, the present day turbochargers have failed to efficiently control the flow of motive gases through the turbine. Presently, there are basically two methods used for controlling pressure through the turbine. The first of these methods is by careful sizing of the turbine and turbine nozzle area so that at maximum engine operating speed and load the desired boost pressure will not be exceeded. The disadvantage of this method is that at low engine speeds the available boost pressure is limited and the response to demand is slow. The second method used for controlling the pressure through the turbine is the use of a "wastegate" in conjunction with a turbine nozzle sized to produce excessive turbine power at maximum engine speed and load. In this method, when the predetermined boost pressure is reached, the "wastegate" opens and bypasses a portion of the exhaust gases. While this method increases the available boost at the lower engine speeds and provides improvements in response, it is quite inefficient in that the bypassed, high pressure exhaust gas is simply wasted at the expense of increased engine back pressure. Additionally, at part load, when the turbocharger is essentially inoperative, the small nozzle area acts as a restriction to the exhaust and causes an increase in the pumping loop loss.
Therefore, a need has arisen for a turbocharger which can be efficiently operated to turbocharge both small and large internal combustion engines. The need is for a turbocharger having a bearing system which eliminates the problems heretofore experienced by continuous engine oil lubricated bearings and makes the most efficient use of the motive gases for driving the turbocharger turbine. Further, the bearing assemblies supporting the rotation of the compressor and turbine must facilitate the reduction of the required compressor and turbine rotor clearances.